1. Field of the Invention
The present invention relates to a vacuum valve having contact material that gives a stable contact resistance characteristic and current breaking characteristic and more particularly that has an excellent current breaking characteristic on interrupting.
2. Description of the Related Art
The contacts of a vacuum valve whereby current breaking is performed in hard vacuum by utilizing arc diffusion in vacuum comprise two contacts, namely, a fixed and a movable contact, facing each other.
Apart from the three fundamentals of large current breaking performance(i.e. current interrupter characteristic, hereinafter sometimes called breaking performance ), voltage-withstanding performance and anti-welding performance, the erosion characteristic of the contacts is an important requirement of a vacuum circuit breaker.
However, since some of these requirements are contradictory, it is not possible to satisfy all the requirements using a single type of metal. For this reason, in many contact materials that are practically employed, the contact material is selected to match the specific application such as for example large current use or high voltage withstanding ability by a combination of two or more elements that mutually compensate for each other's deficiencies. Although vacuum valves have been developed having excellent characteristics in specific applications, the present situation is that vacuum valves have not yet been developed that fully satisfy increasingly demanding requirements.
For example, for contacts intended for large current breaking performance, Cu--Cr alloy (see issued Japanese patent number Sho. 45-35101) containing about 50 weight % of Cr is known. This alloy shows benefits including that the Cr itself maintains practically the same vapor pressure characteristic as Cu and in addition shows a strong gas getter action, enabling a high-voltage and large-current breaking performance to be achieved. That is, Cu--Cr alloy is frequently used for contacts in which high withstand-voltage performance and large current breaking performance are combined.
Since in this alloy the highly active Cr is employed, in manufacture of the contact material (sintering step etc.) and in processing this contact material to produce the contact pieces, manufacture is conducted with especial care in regard to selection of the raw material powder, admixture of impurities, and control of the atmosphere etc. However, a perfect technique for supplying contact material combining simultaneously the voltage-withstanding performance and contact resistance characteristics required in order to obtain a vacuum valve with both high voltage-withstanding performance and large-capacity circuit breaking has not necessarily been achieved.
As a result of studying improvements in the composition of CuCr contacts, the present inventors became able to provide a vacuum valve equipped with excellent contact pieces combining voltage-withstanding performance and contact resistance characteristics.
The chief feature of CuCr contacts is that the vapor pressures of these two [elements] approximate to each other at high temperature; even after breaking they display comparatively smooth surface damage characteristics and exhibit stable electrical characteristics.
However, in recent years application to even larger current breaking or to circuits in which even higher voltage can be applied has become commonplace and severe wear and stubborn welding of the contacts are being experienced. Vacuum valve contacts that have suffered abnormal damage or wear due to breaking experience abnormal increase in contact resistance and/or rise in temperature when a steady current is next switched on or off and show impairment of voltage-withstanding ability. Abnormal damage or wear must therefore be suppressed to the maximum extent possible.
Investigations have revealed that the contact characteristics of CuCr alloys depend on variations of the Cr content of the alloy, the particle size distribution of the Cr particles, the degree of Cr segregation, and the level of voids present in the alloy. However, notwithstanding advances in optimisation in regard to the above, variability(i.e. dispersion) of the voltage-withstanding performance and contact resistance characteristic is experienced under the conditions of use described above that obtain in recent years, and in order to satisfy a given level of breaking performance it has become necessary for vacuum valves to have both of these characteristics.